A GEPON system is comprised of a station optical line terminating device (OLT: Optical Line Terminal) disposed in a center station, an optical divider for dividing a transmission line into up to 32 lines, and subscriber optical network terminating devices each disposed in a member's house.
In the GEPON system, a wavelength of 1,310 nm is assigned to an uplink digital data signal which is transmitted from each subscriber optical network terminating device to the station optical line terminating device, a wavelength of 1,490 nm is assigned to a downlink digital data signal (including a digital sound signal) transmitted from the station optical line terminating device to each subscriber optical network terminating device, and a wavelength of 1,550 nm is assigned to a downlink video signal (including an analog video signal).
Furthermore, it has been examined conventionally with an eye towards future technological innovation that in the GEPON system, a wavelength band of 1,565 nm or longer is assigned as the wavelengths of digital data signals which are transmitted at a transmission rate of 10 gigabits/second. Furthermore, the wavelength band of 1,565 nm or longer is used also as a light wavelength band for examining a disconnection of a line optical fiber connected between the station optical line terminating device and each subscriber optical network terminating device.
Thus, in the GEPON system, a wavelength division multiplexing method (WDM: Wavelength Division Multiplexing) of assigning a plurality of wavelengths is used to carry out single-core bidirectional optical communications via which an uplink wavelength and a downlink wavelength are transmitted by using a single optical fiber.
However, in the GEPON system, it is necessary to provide a light wavelength band limiting filter for interference prevention for the purpose that each subscriber optical network terminating device does not cause interference (cross talk) between the wavelength of the downlink digital data signal and the wavelength of the downlink video signal.
More specifically, the optical transmission/reception module mounted in each subscriber optical network terminating device needs to include a light wavelength band limiting filter for interference prevention.
Typically, the light wavelength band limiting ability of the light wavelength band limiting filter greatly depends on the light incidence angle to the filter, as shown in FIG. 16.
More specifically, when the number of components having different light incidence angles increases, the light wavelength band limiting filter exhibits a combined light wavelength band limiting ability according to the components having different light incidence angles.
Particularly, in a case in which the wavelength interval between received wavelength bands is narrow, while a light signal having a wavelength band which is desired originally to pass through the filter is blocked, a wavelength band which is desired to be blocked is allowed to pass through the filter. The light wavelength band limiting filter thus cannot sufficiently exhibit its light wavelength band limiting ability.
Therefore, a light wavelength band limiting filter having a high required ability to prevent interference (cross talk) of an optical reception signal is used in a collimating optical system which can reduce light incidence angle components in many cases, but is used rarely in a diffusing optical system. However, the collimating optical system has a complicated structure, as will be mentioned below.
The angle of incidence of light to a light wavelength band limiting filter depends on a displacement caused by an installation angle with respect to an optical fiber a displacement caused by an installation angle with respect to a wavelength division multiplexing filter (WDMF: Wavelength Division Multiplexing Filter), a displacement caused by an installation angle of the light wavelength band limiting filter itself, etc.
Because it is impossible to measure the angle of incidence of light to the light wavelength band limiting filter, it is necessary to design the optical transmission/reception module in consideration of the amount of angular displacement within the limits of design assurance in the structural design.
Hereafter, the characteristics of a collimating optical system will be explained.
A collimating optical system has a characteristic of making the angle of divergence of an optical beam (Beam Divergence Angle) be about zero degrees, as shown in FIG. 17.
At this time, the angle component of a light ray incident on a light wavelength band limiting filter depends only on the angle of incidence (AOI: Angle of Incident) of the light ray.
Furthermore, in order to replace the inside of a light module with a collimating optical system (collimate optical system), it is necessary to connect the optical fiber to collimating optical equipment or the like.
Typical collimating optical equipment is comprised of an optical fiber and a lens, and it is necessary to align a positional relationship between the light emitting surface of the optical fiber and the lens with the focal length of the lens.
Therefore, a collimating optical system is complicated compared with a diffusing optical system, and its component count increases and has an expensive and complicated structure.
Next, the characteristics of a diffusing optical system will be explained.
It is generally known that a diffused light beam emitted from the end surface of an optical fiber to the inside of a light module has an angle of divergence showing a Gaussian beam shape, as shown in FIGS. 18 and 19, the angle of divergence of the optical beam can be given by a half angle ξ0=5.6 degrees at which the intensity of the light ray is 1/e2 times the maximum intensity.
Therefore, as the angle component of an optical beam incident on a light wavelength band limiting filter in a diffusing optical system, two angles: the angle of divergence of the optical beam, and the angle of incidence AOI of the center of the light ray must be taken into consideration.
More specifically, in a diffusing optical system, because the angle component of an optical beam incident on a light wavelength band limiting filter has an angle of divergence even if the angle of incidence AOI of the center of the light ray can be reduced to zero degrees, it is difficult for the diffusing optical system to sufficiently exhibit a light wavelength band limiting ability.
Next, the internal structure of an optical transmission/reception module will be explained.
Typically, the end surface of a fiber ferrule disposed within the optical transmission/reception module is not perpendicular to the fiber core of an optical fiber (a region in which light is confined inside the optical fiber), as shown in FIG. 20, but is slanted at an angle of 6 to 8 degrees.
The reason why the end surface of the fiber ferrule is slanted is because if the end surface is formed to be perpendicular to the fiber core of the optical fiber, light transmitted from a station optical line terminating device reflected by the end surface and a requirement about the amount of reflection attenuation which is required in a GEPON system (a requirement showing that a subscriber optical network terminating device must not reflect a certain amount or more of light to a station optical line terminating device) cannot be satisfied.
Because the end surface of the fiber ferrule is thus slanted, the light ray is slantwise emitted from the end surface of the fiber ferrule according to the Snell's law. For example, in a case in which the end surface of the fiber ferrule is slanted at an angle of 8 degrees, the light ray slanted at an angle of about 3.8 degrees is emitted from the end surface of the fiber ferrule.
By adjusting the slope of the angle AOI of the center of this light ray with respect to the optical axis of the wavelength division multiplexing filter not at 45 degrees, but at 48.8 degrees or 41.2 degrees (45±3.8 degrees), for example, the optical transmission/reception module can be constructed in such a way that the light ray can be incident perpendicularly on the filter, the optical transmission module, and the optical reception module disposed in the optical transmission/reception module.
Although in a GEPON system an optical transmission/reception module mounted in a subscriber optical network terminating device needs to include a light wavelength band limiting filter for interference prevention, as mentioned above, an optical transmission/reception module disclosed by patent reference 1 mentioned below carries out demultiplexing and multiplexing of light signals having a plurality of wavelengths by using a wavelength division multiplexing filter to implement single-core bidirectional optical communications.
However, because a lens coupling optical element is only connected between the wavelength division multiplexing filter and the optical fiber in this optical transmission/reception module, the optical transmission/reception module cannot be applied to a GEPON system in which a light wavelength exists adjacent to each of the light wavelength of a downlink digital data signal and the light wavelength of a signal for video image.
FIG. 21 is an explanatory drawing showing the pass characteristic of a light wavelength band limiting filter of an optical reception signal in a diffusing optical system (divergence optical system) as disclosed in the patent reference 1.
In a case in which a light wavelength band limiting filter for interference prevention disposed within an optical transmission/reception module mounted in a subscriber optical network terminating device is used in a diffusing optical system (divergence optical system), the optical transmission/reception module cannot sufficiently exhibit its light wavelength band limiting ability for short wavelengths (a function of attenuating a wavelength band of λ1-α), as can be seen from FIG. 21.
Furthermore, it cannot be said that the optical transmission/reception module can exhibit its light wavelength band limiting ability even for long wavelengths (a function of attenuating a wavelength band of λ1+β with a margin.
Patent reference 2 which be mentioned below discloses an optical transmission/reception module using a light wavelength band limiting filter and a collimating optical system.
In a case in which a light wavelength band limiting filter for interference prevention disposed within an optical transmission/reception module mounted in a subscriber optical network terminating device is used in a collimating optical system (collimate optical system), the optical transmission/reception module can sufficiently exhibit its light wavelength band limiting ability for short wavelengths (a function of attenuating a wavelength band of λ1-α), as can be seen from FIG. 22.
Furthermore, the optical transmission/reception module can exhibit its light wavelength band limiting ability even for long wavelengths (a function of attenuating a wavelength band of λ1+β with a margin.
It can be seen from the above description that the use of a complicated collimating optical system makes it easy to satisfy the light wavelength band limiting ability of a light wavelength band limiting filter.
In order to maintain the light wavelength band limiting ability of a light wavelength band limiting filter to prevent interference (cross talk) of an optical reception signal, collimating optical equipment or the like in which the inside of an optical transmission/reception module is replaced by a collimating optical system (collimate optical system) is disposed and the light wavelength band limiting filter is used in the collimating optical system. As a result, the light wavelength band limiting filter can exhibit the light wavelength band ability.
A problem with the collimating optical system is, however, that the component count increases and the collimating optical system has a complicated structure.
Furthermore, in a diffusing optical system, if the angle of incidence of light to a light wavelength band limiting filter can be managed with a high degree of precision, the light wavelength band limiting ability of the light wavelength band limiting filter can be improved.
Although there are a method of using an inclined lightguide, as disclosed in patent references 3 which will be mentioned below, and a method of using a decentered lens, as disclosed in patent references 4 which will be mentioned below, these methods need a complicated internal structure and are not methods of adjusting the angle of incidence of light with a simple structure.